The traditional two-stage method for obtaining nitrites from the corresponding aldehydes consists in the oximation of an aldehyde with hydroxylamine followed by dehydration of the resulting aldoxime using various dehydrating agents such as acetic anhydride, phosphorus pentoxide, dicyclohexylcarbodiimide, etc. (Scheme 1). See: G. Tennant, "Imines, Nitrones, Nitriles, and Isocyanides" in: Comprehensive Organic Chemistry, ed. I. O. Sutherland. Pergamon Press, Oxford, New York, Toronto, Sydney, Paris, Frankfurt, 1979, Vol. 2, pp. 385-590 (especially pp.533-537), and references therein. Recently, alkali metal hydroxides were patented as the dehydrating agents for this process (M. Oku and Y. Fujikura, U.S. Pat. No. 5,457,222 and U.S. Pat. No. 5,514,830). ##STR1##
Modifications of this general method exist which allow "one-pot" preparation of nitrites, without the separation of intermediate oxime (see G. A. Olah and T. Keumi, Synthesis, 1979, pp.112-113, and references therein).
Also, methods are known based on the intermediacy of other N-substituted azomethine derivatives of aldehydes, such as 0-2,4-dinitrophenyl oximes (M. J. Miller and G. M. Loudon, J. Org. Chem., 1975, vol. 40, pp. 126-127), alkylideneaminopyridones (A. R. Katritzky and P. Molina-Buendia, J. Chem. Soc. Perkin I, 1979, pp.1957-1960), etc., where the intermediates are converted into nitrites by the non-oxidative methods.
A number of methods are based on the oxidative conversion of N-substituted azomethine derivatives of aldehydes. For example (Scheme 2a), N-t-butyl benzaldimine was oxidized into benzonitrile by dilsopropyl peroxydicarbonate DiPPDC (H. Ohta and K. Tokumaru, J. Chem. Soc. Chem. Commun., 1970, p. 1601). Dimethylhydrazones (Scheme 2b) were oxidized to nitrites with meta-chloroperbenzoic acid (MCPBA), or in lower yields with hydrogen peroxide in the presence of selenium compounds (S. B. Said et al., Synthesis, 1989, pp. 223-224). Certain types of aromatic dimethylhydrazones can be oxidized with methanolic hydrogen peroxide without catalyst (R. F. Smith et al, J. Org. Chem., 1966, vol. 31, pp. 4100-4102). Reactions of this type were reviewed by J. Mlochowski in Chem. Papers, 1998, vol. 52(1), pp. 45-51 and by J. Mlochowski and S. B. Said in: Polish Journal of Chemistry, 1992, vol. 66, pp. 1901-1928. ##STR2##
All the above shown methods have a mutual disadvantage, namely the use of relatively expensive hydroxylamine, dimethylhydrazine, etc. Methods based on the use of inexpensive ammonia are more attractive from the practical standpoint. The latter methods involve the formation of aldimine intermediate followed by its dehydrogenation or oxidative dehydrogenation according to Scheme 3 (ammoxidation). ##STR3## Practical value of these methods depends largely on the reagent used for the oxidative dehydrogenation. For instance, there is a method based on the use of potassium persulfate in the presence of nickel salts and aqueous ammonia (S. Yamazaki and Y. Yamazaki, Chem. Lett., 1990, pp. 571-574). The procedure gives 21-76% yields of aromatic and unsaturated nitrites from the corresponding aldehydes, including a 48% yield of geranyl nitrile from citral (Scheme 4). ##STR4##
Practical applicability of this method is restricted mostly because it requires 1.5-2.0 moles of potassium persulfate per one mole of the aldehyde, which means 540.6 weight parts of the persulfate per 152.2 weight parts of citral.
In a number of ammoxidation procedures, air or oxygen are used as dehydrogenating agents for the aldimine intermediates according to Scheme 3. These procedures can be classified into two groups: 1) gas phase/high temperature, and 2) liquid phase/low temperature processes.
In a Group 1 process, propanal is converted into a mixture containing some propionitrile and acrylonitrile together with acetonitrile, aldehydes, and other by-products by a reaction with gaseous ammonia and air at 460.degree. C. over Sn/Sb oxide catalyst (M. Cathala, et al., Bull. Soc. chim. France, 1979, pp. 173-178). In another example (R. J. Card and J. L. Schmitt, J. Org. Chem., 1981, vol. 46, pp. 754-757), benzonitrile, p-methoxybenzonitrile, and octanenitrile are obtained in good yields by passing a stream of gaseous ammonia containing 5 mol % of the corresponding aldehyde through a catalyst bed (3 cm of Cu/alumina) at 325.degree. C. According to Brit. Pat. 709,337, acrolein is converted into acrylonitrile in a gas phase reaction with air/ammonia/steam/ nitrogen mixture over molybdenum catalysts at 285-445.degree. C.
A major disadvantage of the Group 1 processes is the high temperature which can affect thermally labile aldehydes and nitrites. For example, according to G. Ohloff, Tetrahedron Lett., 1960, p. 10-14, citral undergoes various isomerization and cyclization reactions already at 130-205.degree. C. Another disadvantage of the Group 1 processes is the use of large excess of ammonia.
In Group 2 methods, reactions of aldehydes with ammonia and oxygen occur in the liquid phase in the presence of copper compounds. Again a significant excess of ammonia is used, and the reaction mixture is strongly diluted with a solvent. W. Brackman and P. J. Smit (Rec. Trav. Chim., 1963, vol. 82(8), pp. 757-762) reported a method for obtaining nitrites by treating aldehydes (0.05 moles) with oxygen at 30.degree. C. in 100 ml of methanol containing 0.4 moles (eight-fold excess) of ammonia, 0.03 moles of sodium methoxide, and 4 mmoles of copper(II) chloride. A detailed study of the reaction using benzaldehyde as starting material was conducted by A. Misono et al. (Bull. Soc. Chem. Japan, 1967, vol. 40, pp. 912-919) at even stronger dilution: 0.03 moles of benzaldehyde and about 0.09 moles of ammonia in about 100 ml of methanol. A process for ammoxidation of acrolein comprising pretreatment of a copper salt with ammonia followed by contacting the obtained complex with acrolein in the presence of oxygen was patented in U.S. Pat. No. 4,202,837. Best yields of acrylonitrile 29-40% were obtained with a molar ratio acrolein:CuBr=1.05-1.2 (examples 2 and 5) which means over twofold excess of the catalyst by weight, and this process is stoichiometric rather than catalytic.
Various known syntheses of nitrites are summarized in the most recent comprehensive reviews: M. North, "General methods and aliphatic nitrites" in: Comprehensive Organic Functional Group Transformations. Ed. A. Katrizky et al.; Elsevier: Oxford, UK; 1995, Vol. 3, pp. 610-640, 733-856; M. J. Kiefel, ".alpha.,.beta.-Unsaturated and aryl nitrites", ibid., pp. 641-676, 733-856.